We have focused our studies on an analysis of the pathogenesis of Experimental Allergic Encephalomyelitis (EAE), an animal model for the human demyelinating disease, multiple sclerosis. The cytokine, IL-12, which is produced by monocytes and dendritic cells, has been shown to play a critical role in the generation of the subpopulation of T lymphocytes (T helper 1[Th1] cells) which mediate EAE. Our studies have demonstrated that the successful differentiation and/or function of autoimmune effector cells is dependent on their ability to respond to quantities of IL-12 that are available in the microenvironment during critical time points. The interleukin (IL)-12 receptor beta2 subunit is the key molecule involved in maintaining IL-12 responsiveness and controlling Th1 lineage commitment. Both IL-12 and interferon-gamma were shown to play separate as well as complementary roles in regulating IL-12 receptor beta2 expression on antigen-specific CD4+ T cells. The B10.S strain of mice is resistant to the development of EAE and was shown to have an antigen-specific defect in the expression of the IL-12 receptor beta2 subunit. Defective expression was secondary to a failure of the MBP-specific Th1 cells to express the membrane antigen, CD40 ligand, which is required for the T cell mediated induction of IL-12 production by macrophages/dendritic cells. IL-12 receptor expression as well as the capacity of Th1 cells from B10.S mice to induce EAE could be restored by the addition of IL-12. As these studies suggest that the development of immunotherapies that target the IL-12 receptor may be useful in the treatment of autoimmune diseases, we have initiated an extensive analysis of the regulation of IL-12 receptor expression by soluble factors and cell associated molecules. IL-18 was originally described as a cytokine which induced interferon- gamma production by established Th1 cells in an IL-12 independent manner. However, subsequent studies demonstrated that exogenous IL-18 in the absence of IL-12 failed to drive Th1 differentiation of na?ve T cells and induced interferon-gamma from established Th1 cells only in combination with IL-12. We have demonstrated that the primary effect of IL-18 on Th1 differentiation is mediated by its capacity to directly upregulate IL-12 receptor expression which thereby enhances IL-12 mediated signaling. The enhancement of IL-12 receptor expression by IL- 18 may be particularly important for the differentiation of foreign antigen- or autoantigen-specific Th1 cells when the stimulatory concentration of IL-12 in the microenvironment is just below the threshold required for Th1 development. The other major factors which we have identified as important in regulation of IL-12 receptor expression are costimulatory signals delivered to the antigen-specific T cells by interactions of the CD80/CD86 molecules on antigen presenting cells with CD28 on the T cell. We have demonstrated that costimulation is required for production of both IL-2 and IL-12 and that both of these cytokines are critical for the ability of T cells to express IL-12 receptors and to differentiate into pathogenic autoreactive T cells. - Autoimmune disease, multiple sclerosis, central nervous system, experimental allergic encephalomyelitis, T helper 1 lymphocytes